To suppress vibration of the distal end of a mufti-link robot, an axis torsion angular velocity (the difference between a link angular velocity and a motor angular velocity) needs to be estimated from a motor angular velocity measured by an encoder installed in a motor that drives respective axes, and be fed back to a motor angular velocity control system. In the estimation, it is required that a nonlinear observer based on a nonlinear dynamic model takes into account elastic joints, and the nonlinear interference forces acting between the links.
To realize such a mufti-input/output nonlinear observer, an accurate dynamic model needs to be built, and robustness to variations in payload and friction-torque is required. Conventionally, an approximative observer based on a one-input/output linear model that performs “disturbance estimation from another link and compensation in the elastic joint model at each one link” is normally used so as to reduce the amount of calculation in control operations.
As CPUs and memories have improved in performance in recent years, a robot control device can now have the benefit of increased computing power and large memory capacities.
However, since such an observer is based on approximation, the vibration suppression effect is not robust, and on top of that, the engineering costs required for implementing control laws and adjusting control gains are enormous.
Also, as angular velocity sensors can be readily obtained these days, it is possible to suppress vibration by mounting an angular velocity sensor on each of the links of a robot arm, directly measuring axis torsion angular velocities, and performing feedback. However, this technique requires a larger number of wirings, resulting in a problem such as a high cost in some cases.
Furthermore, in a vibration suppression control system that performs feedback of axis torsion angular velocities, there is a problem of a decrease in the damping effect of the vibration suppression control when the end effector load is small or has low inertia, regardless of whether the axis torsion angular velocities are estimated by a state observer or are directly measured.